1. Field of the Invention
The present invention relates to locating leaks in fuel cell devices. In particular, the invention provides a method and apparatus for locating internal transfer leaks in a solid polymer fuel cell stack.
2. Background of the Invention
Electrochemical fuel cells convert reactants, namely fuel and oxidant, to generate electric power and reaction products. Electrochemical fuel cells generally employ an electrolyte disposed between two electrodes, namely a cathode and an anode. The electrodes generally each comprise a porous, electrically conductive sheet material and an electrocatalyst disposed at the interface between the electrolyte and the electrode layers to induce the desired electrochemical reactions. The location of the electrocatalyst generally defines the electrochemically active area.
Solid polymer fuel cells typically employ a membrane electrode assembly (xe2x80x9cMEAxe2x80x9d) consisting of a solid polymer electrolyte or ion exchange membrane disposed between two electrode layers. The membrane, in addition to being ion conductive (typically proton conductive) material, also acts as a barrier for isolating the reactant (i.e. fuel and oxidant) streams from each other.
The MEA is typically interposed between two separator plates, which are substantially impermeable to the reactant fluid streams, to form a fuel cell assembly. The plates act as current collectors, provide support for the adjacent electrodes, and typically contain flow field channels for supplying reactants to the MEA or circulating coolant. The plates, which include the flow field channels, are typically known as flow field plates. The fuel cell assembly is typically compressed to ensure good electrical contact between the plates and the electrodes, as well as good sealing between fuel cell components. A plurality of fuel cell assemblies may be combined electrically, in series or in parallel, to form a fuel cell stack. In a fuel cell stack, a plate may be shared between two adjacent fuel cell assemblies, in which case the plate also separates the fluid streams of the two adjacent fuel cell assemblies. Such plates are commonly referred to as bipolar plates and may have flow channels for directing fuel and oxidant, or a reactant and coolant, on each major surface, respectively.
The fuel stream that is supplied to the anode separator plate typically comprises hydrogen. For example, the fuel stream may be a gas such as substantially pure hydrogen or a reformate stream containing hydrogen. Alternatively, a liquid fuel stream such as aqueous methanol may be used. The oxidant stream, which is supplied to the cathode separator plate, typically comprises oxygen, such as substantially pure oxygen, or a dilute oxygen stream such as air.
The electrochemical reactions in a solid polymer fuel cell are generally exothermic. Accordingly, a coolant is typically also used to control the temperature within a fuel cell assembly to prevent overheating. Conventional fuels cells employ a liquid, such as, for example, water to act as a coolant. In conventional fuel cells, the coolant stream is fluidly isolated from the reactant streams.
Thus, conventional fuel cells typically employ three fluid streams, namely fuel, oxidant, and coolant streams, which are fluidly isolated from one another. See, for example, U.S. Pat. No. 5,284,718 and U.S. Pat. No. 5,230,966, which are incorporated herein by reference in their entirety.
Fluid isolation is important for several reasons. One reason for fluidly isolating the fuel and oxidant streams from one another in a fuel cell is that hydrogen and oxygen are particularly reactive with each other. Accordingly, the membrane and plates are, therefore, substantially impermeable to hydrogen and oxygen.
One reason for fluidly isolating the coolant fluid from the reactant fluids is to prevent dilution and contamination of the reactant streams. Indeed, water, which is typically used as a coolant, may cause flooding in the reactant fluid passages that prevents the reactants from reaching the electrochemically active membrane-electrode interface. It is also undesirable for the reactant streams to leak into the coolant stream because this reduces operating efficiency as the leaked reactants are not used to generate electrical power. One reason for preventing leakage of any of the fluids to the surrounding atmosphere is the general negative impact such leakage can have on fuel cell stack safety, performance and longevity.
Locating the source of internal transfer leaks has been found to be problematic. Once an internal transfer leak has been detected within a fuel cell stack (which is typically detected through the constant monitoring of the exhaust streams), locating the source of the leak is typically accomplished by disassembling the fuel cell stack into its constituent parts and testing each fuel cell individually. Such method is time consuming and, consequently, expensive. Furthermore, because the disassembling and individual fuel cell testing process can cause further damage/defects to the stack, such method can result in a worsening of a fuel cell stack fluid integrity.
Accordingly, there is a general need for a method and apparatus for locating internal transfer leaks which does not require a fuel cell stack to be disassembled into its constituent parts and each part being tested individually.
The invention provides a method for locating an internal transfer leak in a fuel cell stack. The method comprises:
a) applying a substantially constant gas pressure difference between a first fluid stream passage and a second fluid stream passage, wherein gas pressure in the second fluid stream passage is higher than gas pressure in the first fluid stream passage;
b) supplying a test gas to the second fluid stream passage;
c) supplying a test liquid to the first fluid stream passage; and
d) measuring a parameter indicative of flow rate of the test gas exiting the first fluid stream passage as the test liquid fills the first fluid stream passage.
The method may further comprise the step of ascertaining the position of the test liquid inside the first fluid stream passage. In one embodiment, this step comprises splitting a flow of test liquid so as to supply the test liquid to the first fluid stream passage and to a level indicator. As a result, the position of the test liquid within the level indicator is indicative of the position of the test liquid inside the first fluid stream passage.
The method may further comprise positioning the fuel cell stack so that individual fuel cell assemblies are aligned along substantially vertically successive horizontal planes.
In an embodiment of the method, the fluid stream passages are reactant stream passages. In another embodiment of the invention, one fluid stream passage is a reactant stream passage and the other fluid stream passage is a coolant stream passage.
In an embodiment of the method, the parameter indicative of flow rate is flow rate of the test gas. For example, in an embodiment where the test gas is air, the method may comprise measuring the flow rate of air exiting the first fluid stream passage as the test liquid fills the first fluid stream passage.
In an alternative embodiment of the method, the parameter indicative of flow rate of the test gas is concentration of the test gas, or component thereof. For example, in an embodiment where the test gas is hydrogen, the method may comprise measuring hydrogen concentration in the air exiting the first fluid stream passage as the test liquid fills the first fluid stream passage.
In an alternative embodiment of the method, the parameter indicative of flow rate of the test gas is flow rate of all gases. For example, in an embodiment where the test gas is hydrogen, the method may comprise measuring flow rate of all gases exiting the first fluid stream passage as the test liquid fills the first fluid stream passage.
In an embodiment of the method, the gas pressure inside the second fluid stream passage is kept substantially constant.
In an embodiment of the method, the gas pressure inside the first fluid stream passage is kept substantially constant at ambient atmospheric pressure.
The invention also provides a further method for locating an internal transfer leak between an oxidant stream passage and a fuel stream passage of a fuel cell stack. The method comprises:
a) positioning the fuel cell stack so that water directed to the fuel stream passage fills channels of fuel flow field plates in successive individual fuel cell assemblies;
b) directing air to the oxidant stream passage;
c) maintaining gas pressure inside the oxidant stream passage at a substantially constant value greater than ambient atmospheric pressure;
d) maintaining gas pressure inside the fuel stream passage substantially constant at ambient atmospheric pressure;
e) directing water to the fuel stream passage;
f) measuring flow rate of air exiting the fuel stream passage as the water fills the fuel stream passage; and
g) ascertaining a water level position inside the fuel stream passage as the water fills the fuel stream passage.
The further method may further comprise relating the flow rate of air measured to the water level position ascertained as the water fills the fuel stream passage.
In an embodiment of the further method, the step of ascertaining the water level position comprises splitting an input flow of water so as to supply the water to the fuel stream passage and to a level indicator, wherein a first water level within the level indicator corresponds to a second water level inside the fuel stream passage.
The invention also provides an apparatus for locating an internal transfer leak in a fuel cell stack. The apparatus comprises:
a) a liquid supply system adapted to supply a test liquid to a first fluid stream passage;
b) a gas supply system adapted to supply a test gas to a second fluid stream passage; and
c) a measuring instrument adapted to measure a parameter indicative of flow rate of the test gas exiting the first fluid stream passage as the test liquid fills the first fluid stream passage.
The apparatus may further comprise a regulating system adapted to maintain a substantially constant gas pressure difference between the first fluid stream passage and the second fluid stream passage, wherein gas pressure in the second fluid stream passage is higher than gas pressure in the first fluid stream passage.
In one embodiment of the apparatus, the regulating system may be adapted to maintain the gas pressure in the second fluid stream passage substantially constant. In this embodiment, the regulating system may comprise:
a) a first entry valve, adapted to regulate flow of the test gas to the second fluid stream passage;
b) a relief valve, adapted to allow the test gas to escape from the second fluid stream passage; and
c) a pressure transducer, adapted to measure the gas pressure within the second fluid stream passage.
In an alternative embodiment of the apparatus, the regulating system may be adapted to maintain the gas pressure in the first fluid stream passage substantially constant at ambient atmospheric pressure. In this embodiment, the regulating system may comprise an atmospheric isolator, securable to an outlet of the first fluid stream passage and adapted to maintain the gas pressure within the first fluid stream passage substantially constant at ambient atmospheric pressure.
In an embodiment of the apparatus, the measuring instrument may be a gas flow meter. For example, in an embodiment where the test gas is air, the measuring instrument could be an air flow meter.
In an alternative embodiment of the apparatus, the measuring instrument may be an emission analyzer adapted to measure a concentration of the test gas or component thereof. For example, in an embodiment where the test gas is hydrogen, the measuring instrument could be a sensor able to detect hydrogen concentration in air.
In an alternative embodiment of the apparatus, the apparatus may further comprise a level indicator fluidly connected to the liquid supply system and indicative of position of the test liquid inside the first fluid stream passage.
In an embodiment of the apparatus, the level indicator may comprise:
a) a Y-shape connector, adapted to:
i) receive a flow of test liquid from the liquid supply system, and
ii) direct the flow of the test liquid:
a. to the first fluid stream passage, and
b. to a container;
b) the container, adapted so that position of the test liquid within the container is indicative of position of the test liquid inside the first fluid stream passage.
Many specific details of certain embodiments of the invention are set forth in the detailed description below to provide a thorough understanding of such embodiments. One skilled in the art, however, will understand that the present invention may have additional embodiments, or may be practiced without several of the details described.